The present invention relates to instrumentation amplifier circuits. More particularly, the present invention relates to an instrumentation amplifier configured to obtain high common mode rejection.
The demand for improved instrumentation amplifier circuits for high-precision data acquisition and instrumentation applications, such as multi-channel data acquisition systems, current shunt monitors, and industrial or physiological sensors, continues to increase. Instrumentation amplifier circuits are generally designed to amplify the difference between two voltage inputs with a defined gain, wherein a single-ended output is provided which is referenced to a known reference point, for example, ground.
There are a variety of instrumentation amplifier circuits available today. One conventional instrumentation amplifier comprises three op-amps as illustrated in FIG. 1. Instrumentation amplifier 100 comprises two op amps, A1 and A2, which operate as xe2x80x9cgain cellsxe2x80x9d and are suitably connected together so that a differential input signal is applied to the positive inputs of the two op amps, while the outputs of the two op amps are applied as a differential input to a third op amp, A3, that is connected as a difference amplifier. In addition, instrumentation amplifier 100 is generally configured to be linear and have a well-defined gain. To achieve high common mode rejection (CMR), instrumentation amplifier 100 requires the precise matching of resistors R4 through R7. This matching requirement is undesirable because the extremely precise matching of resistors with high manufacturing yields can be difficult and expensive to achieve.
One approach for an instrumentation amplifier which does not require precision resistors to achieve high DC common mode rejection is disclosed by Toumazou and Lidgey in xe2x80x9cNovel Current-Mode Instrumentation Amplifierxe2x80x9d, Electronics Letters, Vol. 25 No. 3, Feb. 2, 1989, and is illustrated in FIG. 2. Instrumentation amplifier 200 comprises a three op-amp configuration wherein unity gain buffers A1 and A2 are each comprised of an op amp and an output stage circuit. Unity gain buffers A1 and A2 are configured to receive a differential input voltage VIN, i.e., the difference between input voltages VIN+ and VINxe2x88x92, at the positive input terminals and create a current mode signal through resistor RIN equal to VIN/RIN. This current mode signal is supplied from the difference in supply currents from the unity gain buffer A1. In addition, instrumentation amplifier 200 includes current mirrors CM1 and CM2 which are configured to mirror the supply currents from both the op amp and the output stage circuit of unity gain buffer A1, the VIN+ unity gain buffer. Only the difference in supply currents, VIN/RIN, flows out of the output of current mirrors CM1 and CM2 and into resistor ROUT. As a result, a voltage is developed equal to VINxc3x97(ROUT/RIN) that can be buffered through unity gain buffer A3 to the output terminal VOUT.
This instrumentation amplifier configuration can provide good DC common mode rejection since only differential signals are passed to the output unity gain buffer A3. However, any AC load current to ground occurring at the negative input terminal of unity gain buffer A1 will be realized as a differential signal and will result in poor AC common mode rejection. This CMR error is a result of the parasitic capacitance Cpar2 from the wiring connections and inherent parasitics in resistor RIN, as well as the input and output stage capacitance of unity gain buffer A1. This parasitic capacitance Cpar2, can create significant degradation in AC common mode rejection at frequencies as low as 60 Hz. Further, this CMR error with respect to the parasitic capacitance Cpar2 at node 2 can be expressed as VOUT=VCMxc3x97(sxc3x97ROUTxc3x97Cpar2).
While other approaches have been disclosed for providing an instrumentation amplifier without precision resistors for providing high DC and AC common mode rejection, these configurations are more complex in design. For example, some of these instrumentation amplifiers include significantly more terminals, such as 18-lead or 20-lead pin configurations. These configurations differ greatly from a more desirable 8-pin configuration, for example, MSOP-8 or SO-8 surface mount packages being preferred.
Another limitation associated with currently available instrumentation amplifiers is the impact of supply voltages. For example, currently available instrumentation amplifiers have great difficulty in providing for large voltage swing capability for input terminals. While it would be highly desirable if these instrumentation amplifiers could provide rail-to-rail voltage swing capabilities, currently available instrumentation amplifiers cannot suitably provide true rail-to-rail voltage swing due to their operating characteristics.
Accordingly, a need exists for an instrumentation amplifier which can provide good AC and DC common mode rejection. In addition, a need exists for an instrumentation amplifier which can provide a very desirable 8-pin configuration to facilitate more desirable package design. Further, a need exists for an instrumentation amplifier configured to provide rail-to-rail voltage swing capability at input, output and reference terminals.
The method and circuit according to the present invention addresses many of the shortcomings of the prior art. In accordance with one aspect of the present invention, an instrumentation amplifier is provided which can provide high common mode rejection without the use of precision resistors. In addition, the instrumentation amplifier can be configured in an 8-pin layout, such as, for example, for an MSOP-8 or SO-8 surface mount package. Moreover, the external gain setting circuits can be configured in various arrangements.
In accordance with another aspect of the present invention, an instrumentation amplifier can be provided which can effectively cancel the differential mode signal created by common mode input signals, such as caused by parasitic capacitances and the like which are detrimental to AC common mode rejection. Accordingly, an exemplary instrumentation amplifier is provided that can exhibit excellent AC as well as DC common mode rejection. In accordance with an exemplary embodiment, an instrumentation amplifier can comprise two pairs of current mirrors configured with two buffers to suitably add the differential current-mode signals and subtract the common current-mode signals of each buffer to thereby cancel the differential mode signal created by common mode input signals, such as those that may be created from the parasitic capacitances within the instrumentation amplifier. In accordance with another exemplary embodiment, the buffers and/or current mirrors can be can be chopper stabilized to further enhance the operation of the instrumentation amplifier.
In accordance with another aspect of the present invention, an instrumentation amplifier can be configured to provide rail-to-rail voltage swing capabilities at the input, output and/or reference terminals. Rail-to-rail voltage swing means to or beyond the power supply rails for inputs, and proximate to the power supply rails for outputs, for example, within approximately 100 mV. In accordance with an exemplary embodiment, an instrumentation amplifier can include a pair of charge pumps which are configured to provide supply voltage to the buffers and current mirrors. In addition, the pair of charge pumps can comprise a positive charge pump and a negative charge pump which can provide an additional voltage beyond the supply voltage, thus facilitating rail-to-rail voltage swing capabilities at the input, output and/or reference terminals.